Graphitic carbon nitride (C3N4)-BiVO4 heterojunctions with various mass ratios of C3N4 and BiVO4 were synthesized by a simple hydrothermal method. High-resolution transmission electron microscopy (HR-TEM) results show that an interface of intimate contact is formed between C3N4 and BiVO4 in heterojunctions. The UV-vis diffuse reflection spectra reveal that the resulting C3N4-BiVO4 heterojunctions exhibit more intensive absorption within the visible light range in comparison with pure C3N4. The photocatalytic tests demonstrate that the resulting C3N4-BiVO4 heterojunctions possess significantly enhanced photocatalytic activities for methylene blue (MB) degradation under visible light irradiation compared with individual C3N4 and BiVO4. The optimum photocatalytic activity of the 0.7 C3N4-0.3 BiVO4 heterojunction is almost 3.5 and 2.8 times higher than those of individual C3N4 and BiVO4, respectively. On the basis of experimental result, a possible photocatalytic mechanism that has superoxide radical species as the mainly active species in photocatalysis is proposed. Additionally, the present study provides a useful strategy to design heterojunction materials with enhanced photocatalytic performance.